The present invention relates to AM broadcast transmitters, and more particularly those that use a pulse width modulation (PWM) modulator to supply voltage to a solid state final amplifier and a driver stage therefor.
The AM transmitter of the present invention has an internally generated subcarrier that is pulse-width modulated by a DC and an audio signal. The width-modulated pulses are amplified by a switching type amplifier. A low-pass filter (pulse-width-filter, PWF) following the amplifier removes the subcarrier and its sideband components and provides amplified DC and audio voltages at its output that are applied to the final power amplifier (PA) and a driver stage. The DC modulation sets the duty factor of the square wave subcarrier to provide a selected DC voltage required for a given RF carrier power, and the audio modulation of the pulse width of the subcarrier provides the magnitude of the audio voltage required for a given percentage AM modulation of said carrier. To obtain a maximally flat audio frequency response, the load presented to the output of the PWF must be a resistance of correct magnitude and be constant with the frequency, and for minimum total harmonic distortion (THD), this load impedance must be linear, i.e., constant, throughout the audio cycle from 100% negative modulation to 100% positive modulation. However, it has been found that as the voltage applied to the PA varies from the carrier DC level to 100% negative modulation, the filter load impedance decreases. This is caused by a number of factors such as: (a) the output capacitance of the PA transistors increases with decreasing collector-emitter voltage, resulting in a lower cut-off frequency (f.sub.t) characteristic, thereby slowing the turn-off of the devices and resulting in higher shoot-through current (this is a very large current spike when momentarily all transistors in the PA bridge are conducting because of overlapping turn-on and turn-off times), and (b) an RF drive transformer for the PA becomes less efficient because the modulated drive from the driver stage does not provide enough magnetizing current in the ferrite core of the transformer. This results in lower RF drive at the trough of modulation. In either case, distortion results, as explained above. It is therefore desirable to maintain a linear load at the output of the PWF.